The recent development of data communications and personal communication systems (PCS) has led to a drastic increase in the traffic in RF transmission. In order to meet this increase, communication systems at millimeter wave frequencies (greater than 25 GHz) are required. The circuits for operation at these high frequencies are generally fabricated using semiconductors with high electron mobility, such as GaAs and related compounds, and are often called Monolithic Microwave Integrated Circuits (MMICs). These MMICs must be mounted in a housing with other components to form a complete module. The requirements for an ideal housing include: [1] universal RF input/output terminals for coaxial and/or waveguide interfaces, [2] hermetically sealed terminals for DC and RF, [3] gold plating for thermal compression bonding, [4] proper cavity design to minimize moding and [5] mounting interface for heat sink attachment.
Since the wavelength of a millimeter wave is short, the requirements for the MMICs fabrication and the tolerance of alignment and dimensions of parts are critical. Hence, a slight deviation of the dimensions or position of parts used in the housing and specifically in connection from the predetermined values may result in poor performance of the entire module. This is particularly true for the RF input and output transitions. In addition to the design and fabrication of MMICs, one of the critical steps for obtaining a high quality millimeter wave module is to provide a precise and reproducible RF transition between the MMICs and connection means attached to the housing.
The requirements for the RF transition include the following: [1] a glass bead directly mated with coaxial connectors, [2] a precisely fabricated probe attached to the bead for proper impedance matching. A transition between a waveguide and microstrip line has been reported in “1988 IEEE MTT-S Digest, pp. 473–474” entitled “Waveguide-to-Microstrip Transitions for Millimeter-Wave Applications” by Yi-Chi SHIH Thuy-Nhung TON and Long Q. BUI, both SHIH and BUI are also the common co-inventors of the present invention. For the method involving waveguide-to-microstrip transition, dimensions of the microstrip line must be controlled precisely and aligned to an aperture in the wall of the housing in order to achieve matched impedance, for example 50 ohms. For reliable operation, the microstrip line part must be secured to the aperture of the housing, which often affects the alignment of the microstrip to the aperture of the housing.
A millimeter wave waveguide launch transition feedthrough was also disclosed in U.S. Pat. No. 5,376,901 entitled “Hermetically Sealed Millimeter Waveguide Launch Transition Feedthrough” granted to Steven S. Chan, Victor J. Watson, Cheng C. Yang and Stuart Kam. An electrically conducting pin with a cylindrical or conical conductive bead head is first formed into a waveguide probe for the transition feedthrough. The transition feedthrough is then mounted in an aperture of a housing with the bead head extending inside an integrated waveguide. Using their method and structure, it is difficult to obtain positional reproducibility of the bead head with respect to the integrated waveguide, especially for applications at millimeter wave frequencies. This is because there is always a gap between the ring and inner wall of the aperture in the housing. Hence, the uniformity of the transition feedthrough in the final modules can not be guaranteed. In addition, the fabrication of the cylindrical or conical waveguide probes is relatively expensive due to the tight requirements in dimensions and position of the central hole.
In order to achieve low cost production of millimeter wave modules, it is preferable to use housings with the same structure and dimensions for different modules. To achieve this, the housings should allow RF input and output to be achieved with either coaxial connector or waveguide connector. The housings should preferably be capable of hermetic sealing in order to isolate the MMICs and components from environmental contaminants.
In U.S. patent application Ser. No. 09/35 1,362, filed by Yi-Chi Shih, Long Q. Bui and Tsuneo C. Shishido on Jul. 12, 1999, now U.S. Pat. No. 6,363,605, a universal conductive housing for different millimeter wave MMICs with a feedthrough has been disclosed. A plate shape waveguide probe, which is symmetrical and fabricated by a micro lithography and etching method, is aligned using a precision alignment tool with respect to a pin of the feedthrough and welded or soldered by a miniature solder. The uniformity and reliability of the waveguide transition has been improved using the structure described in the U.S. Pat. No. 6,363,605. However, since the waveguide probes described in that invention are symmetrical and aligned perpendicular to the major exterior wall of the universal conductive housing and perpendicular to the broad walls of the waveguide, the electric field polarization is always perpendicular to the major exterior wall of the universal conductive housing. Hence, the input/output waveguide interface always forms a 90 degrees angle with respect to the normal of major walls of the universal conductive housing. In many applications, it is very desirable and sometimes necessary to integrate components in-line with the main housing at the waveguide input/output interfaces, i.e. the long axis of the input/output waveguide interface should form a near zero degree angle with respect to the normal of major walls of the universal housing. This requirement thus creates a need to have a new arrangement and structure for the waveguide probe. Furthermore, it is preferable to have a waveguide transition with operating frequency range broader than the previous structure involving symmetrical waveguide probes.